I use shade cloth on both of my greenhouses in the spring and summer months. I remove them in mid-October since the sun is so low in the southern sky, and that low angle doesn't sunburn the tropical plants. One of my greenhouses has semi-transparent panels, so it gets somewhat filtered sunlight regardless. Now that I have wrapped both greenhouses in Solar Blanket bubble wrap, the sun-filtering is further enhanced.
The automotive clear coat can be obtained from probably any auto repair shop and perhaps the auto supplies businesses, like AutoZone carries it as well. Keep in mind that this clear coat would have no shading value, it just (might) extend the lifespan of your panels. You are using it "off-label". If you need your GH shaded, you would have to put some sort of shade-cloth over it.
